You can use \\ && A&B
instead of \\ A&B
. The first two &&
are to skip over the first two columns.

Notes:
- As per Bernard'scmment, the
multirow
package was used to vertically center the first two headers across the two rows.
- As per @AlanMunn's comment,
\cline{3-4}
can be used to draw lines across specified columns.
However, I would strongly recommend you have a look at the booktabs
package for tables:

Code:
\documentclass{article}
\usepackage{multirow}
\begin{document}
\begin{tabular}{|c|l|c|c|}
\hline
\multirow{2}{*}{\bfseries First} &
\multirow{2}{*}{\bfseries Characteristics} &
\multicolumn{2}{|c|}{\bfseries Implemented in}\\ \cline{3-4}
&& A&B \\ \hline
%------
& b & c & d \\
1 & 2 & 3 & 4\\
\end{tabular}
\end{document}
Code: booktabs
\documentclass{article}
\usepackage{booktabs}
\usepackage{multirow}
\begin{document}
\begin{tabular}{c l c c}
\toprule
\multirow{2}{*}{\bfseries First} &
\multirow{2}{*}{\bfseries Characteristics} &
\multicolumn{2}{c}{\bfseries Implemented in}\\ \cmidrule(lr){3-4}
&& A&B \\ \cmidrule(lr){1-4}
%------
& b & c & d \\
1 & 2 & 3 & 4\\
\bottomrule
\end{tabular}
\end{document}
\\ && A&B
instead of\\ A&B
. The first two&&
are to skip over the first two columns. You should also have a look at thebooktabs
package.